The present invention relates to a semiconductor apparatus, and more particularly to a three dimensional mask which can obtain a good edge pattern to improve the photolithographic exposure process.
In the semiconductor industry, photolithographic exposure tools such as steppers and scanners have been used to define patterns in photosensitive material known as photoresist. After photoresist material is spun onto a substrate, an exposure tool repeatedly projects an image of the pattern that is defined on the mask to repeatedly expose the photoresist layer. The properties of the exposed portions of the photoresist layer are altered for subsequent processing steps such as resist development and consecutive substrate etching or implantation.
A mask is typically a transparent plate such as quartz with opaque elements such as a chromium layer on the plate used to define a pattern. A radiation source illuminates the mask according to well-known methods. The radiation transmitted through the mask and exposure tool projection optics forms a diffraction limited latent image of the mask features on the photoresist layer. Further discussion of patterning principles and diffraction limited microlithography can be found on pages 274-276 of VLSI Technology edited by S. M. Sze ((copyright)1983).
FIG. 1 is a schematic cross-sectional view of a mask in accordance with the prior art. Referring to FIG. 1, an opaque chromium layer 104 with an opening pattern 106 is formed on a transparent plate 102 by utilizing etching or printing technology to form a mask 100. Generally, the opening pattern 106 on the mask 100 is a periodical dense pattern. While determining the illumination condition, however, numerical aperture (NA) and coherence "sgr" are simultaneously calculated by an infinite period. When the condition is applied to a finite periodical mask pattern, optical proximity effect (OPE) occurs in the edge of the mask pattern, and pattern distortion and depth of focus (DOF) deficiency are produced. FIG. 2 is a diagram illustrating an exposing pattern on a photoresist layer by a conventional mask. As shown in FIG. 2, the center part 202 of the exposing pattern 200 has the same shape as the pattern on the mask 100. However, in the edge part 204 of the exposing pattern 200, pattern distortion occurs. This results with the line width d of edge pattern being unexpectedly enlarged or shrunk. Unfortunately, the circuit on the chip is therefore readily shorted or disconnected, and the process window of the exposure process is decreased.
The causes of the OPE include optical factors of interference between light beams transmitted through adjacent patterns. By regularly deforming the pattern on the mask, an exact exposing pattern can be obtained, and this process is referred to as xe2x80x9coptical proximity correctionxe2x80x9d (OPC). The OPC process must be adjusted in accordance with the illumination condition, and the OPC mask must be modified with a different illumination source to get a perfect pattern. This results in application limits to the OPC mask.
The present invention provides a three dimensional mask that can improve shape distortion which occurs on the edge pattern and can be applied to different illumination source.
The present invention provides a three dimensional mask adapted for a photolithographic exposure process. The mask comprises a base transparent layer and an opaque layer on the base transparent layer. The opaque layer has an opening pattern that includes an edge opening. A collimating protrusion is located on the opaque layer and surrounds the edge opening. Setting the collimating protrusion improves the interference to get more precise exposure on a photoresist layer.
The present invention also provides a three dimensional mask adapted for a photolithographic exposure process. The mask comprises a base transparent layer and an opaque layer on the base transparent layer. The opaque layer has an opening pattern, and the part of the opaque layer including the opening pattern has a concave surface. The thickness of the opaque layer in the edge of the opening pattern is larger than the thickness of the opaque layer in the center of the opening pattern.
The three dimensional mask of the present invention can improve the shape distortion and depth of focus (DOF) deficiency in the edge pattern caused from the optical proximity effect (OPE) by extending the transmission path of edge pattern to recondition radiation coherence.